A diffraction grating device is a device in which a diffraction grating based on perturbation of refractive index is formed through a predetermined range in a light guide direction in an optical waveguide (e.g., an optical fiber). This diffraction grating device is able to selectively reflect light within a predetermined reflection band out of light guided through the optical waveguide, by the diffraction grating. A multiplexer/demultiplexer module incorporating this diffraction grating device is able to multiplex or demultiplex light by selectively reflecting light in the reflection band by the diffraction grating device and is used, for example, in WDM (Wavelength Division Multiplexing) transmission systems configured to perform optical transmission using wavelength-multiplexed signal light of multiple wavelengths.
In general, the diffraction grating device is provided with the diffraction grating based on refractive index perturbation of a definite period Λ through the predetermined range in the light guide direction in the optical waveguide and this diffraction grating selectively reflects light of wavelength λ satisfying the Bragg condition represented by the equation of λ=2N·Λ, but transmits light of the other wavelengths Here N is an average effective refractive index in the index perturbation region of the optical waveguide.
There arises a problem of group delay difference in the reflection of light in the reflection band by the diffraction grating device of this type. Specifically, reflection of light in the diffraction grating occurs everywhere in the light guide direction in the diffraction grating, so that effective reflection positions vary according to wavelengths, which causes group delays in the reflected light. If the wavelength dependence of the group delays is strong in the reflection band, the waveform of the signal light will deteriorate due to the reflection in the diffraction grating device, so as to cause a reception error easily, which does not permit increase of capacity in the WDM transmission.
Consequently, the diffraction grating devices designed to decrease the group delay difference caused by the reflection of light in the reflection band are described in Document 1 “M. Ibsen, et al., “Optimised square passband fibre Bragg grating filter with in-band flat group delay response,” Electron. Lett., Vol. 34, No. 8, pp.800-802 (1998)” and Document 2 “M. Ibsen, et al., “99.9% Reflectivity Dispersion-less Square-filter Fibre Bragg Gratings for High Speed DWDM Networks,” OFC' 2000, PD21 (2000).”
The diffraction grating device described in Document 1 includes the apodized diffraction grating having a nonuniform amplitude distribution of refractive index perturbation. By setting the origin at the center position of the predetermined range in which the perturbation of refractive index of the diffraction grating is formed, defining the z-axis along the light guide direction with respect to the origin, and expressing the amplitude distribution of the index perturbation of the diffraction grating by a function of a parameter z, the apodized amplitude distribution is expressed by the product of a sinc function and a Gaussian function and is of antero-posterior symmetry with respect to the center on the center position of the predetermined range.
The diffraction grating device described in Document 2 also includes the apodized diffraction grating having the nonuniform amplitude distribution of index perturbation. The amplitude distribution of refractive index perturbation in the diffraction grating device is one designed by the layer peeling technique layer peeling Bragg grating design method). Specifically, the predetermined range in which the refractive index perturbation of the diffraction grating is to be formed, is divided into a number of small zones and refractive indices of the respective small zones are determined in order from one end of the predetermined range so as to achieve desired reflectivity characteristics on the whole of the diffraction grating, thereby designing the amplitude distribution of perturbation of refractive index.